IRMA roof thermal performance
A flat or low slope IRMA roof [inverted roofing membrane assembly] is great for low slope roofing with a parapet, but I don’t see a lot of literature about energy efficiency during service life.
I don’t understand the thermal performance of an IRMA roof during rain events. does the thermal lag of structural concrete mitigate the effect of the water during cold wet days which are just above freezing? during a rain event or melted snow, the water is flowing past the uninsulated membrane, thus cooling the surface of the structure.
Seems like this would be even worse when there is less thermal mass.
then does an IRMA roof with a composite slab or just metal deck perform that much worse than IRMA over a structural concrete deck?
I am curious if there are any WUFI models which explore this?
I’ve done a few google searches and looked at PMRA, dupont, and owens corning websites, Barrett, Henry, AHI websites but came up empty.
Anyone have some relevant literature or data?
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Replies
stamant,
Interesting question. I'll be following this.
stamant,
The IRMA system is seen mostly on large commercial buildings. Below are some additional links to companies and a podcast (transcribed) that you may find useful. IRMA is a pretty pricey option. I don't see why IRMA would be preferable for a design with parapets. I am attaching two views of my own home that has mixed standing seam and low pitch membrane. The dark trims along the top edges of the parapets are the membrane edges.
All roofing metal and membrane is over nailbase and or polyiso as needed. The membrane is not fully adhered, but mechanically fasten with big washers and screws. The next lap of membrane is heat fused to the fastened edge covering the fasteners on the first course. The membrane is also fused to the screwed down drip edge that shows along the parapet's edge. Due to the low pitch I specified, no metal roofing vendors would warrant against water/ice issues for the low pitch roofs. Metal roof is 3.5/12.
The parapet caps are special pvc coated stock that allows the membrane to come up over the parapet which fully seals the interior somewhat like a bathtub. I opted for a completely open low end and gutterless design over scuppers. The IB system has a number of options you should be able to see on their site.
My roof R is +R52 except over the garage (pic) where we went with R-25 or so. Thermally I am fine winter or summer. Membrane warranty is life time for me. Don't want to speculate on how long that is, but we are 8 years into the installation. The surface is slightly faded from new, but I spend more time looking at our views not the roof.
FWIW, I agree that having water pooling under your insulation would not be a good feature. My low pitch roof drains pretty quickly, but sheets of insulation would impede flow severely. Melt water would suck up lots of BTUs on the way out defeating the insulations purpose. The ballast on top also adds to your structural needs though trusses are very efficient for a wide range of loads. Maybe a sort of "cheat sheet" of plastic film under the ballast and over the insulation could alleviate most of the intrusion down below the insulation. However, just another leak to find when it fails.
One pitch point of IRMA roofs IS easy leak repair based on the primary membrane being fully adhered which limits leak migration unlike my fastened membrane. Not sure it is a worthy trade off in a residential application. Leaks are a PITA no matter. At least I can easily inspect the whole roof membrane without having to lift up ballast and insulation.
Here are some links that may lead to other info you can use.
https://eliteroofing.com/inverted-roof-membrane-assembly-irma-or-protected-roof-membrane-assembly-prma-explained/
https://www.hydrotechusa.com/roofing
https://ibroof.com/
https://www.everythingbuildingenvelope.com/advantages-limitations-of-prma-irma-design-configurations-for-roof-covering-systems.htm podcast
I agree with most everything in the podcast; the sources you include don't really get to the meat of my thermal performance question. You won't get an IRMA manufacturer to tout the advantages of a contending system. It doesn't help them sell their product, especially when the IRMA system has real advantages for a large scale commercial building.
To add to the discussion of edge conditions for anyone interested parapets simplify the edge conditions and increase uplift resistance.
https://www.spri.org/wpfb-file/ansi-spri-rp-4-2022-wind-design-standard-for-ballasted-single-ply-roofing-systems-pdf/
https://www.spri.org/wpfb-file/ansi_spri-rp-14-2016-wind-design-standard-for-vegetative-roofing-systems-pdf-2/
leak tracing is easier since leaks don't have a lateral path through insulation and the insulation is not susceptible to damage from moisture.
These advantages don't have a lot to do with thermal performance during service life. It would be great to sneak some data-loggers into the interstitial space above the finished ceilings or in the mechanical area of these large buildings. it might be the case that the times that the IRMA roof has poor performance due to precipitation is not significant compared with the other times where the continuous insulation is working great. it may be that the concern is marginal and academic, but that should be backed up with real data.
They will get colder and thermal performance will be degraded. This is recognized in Europe and called a cold roof, whereas the convention roof build ups with vapor retarder, insulation, cover board, and roof membrane are referred to as warm roofs. There's plenty of ISO / EN standards that provide ways to calculate the R-value loss.
There are notes in some new insulation product data or technical references in the US, but are more in reference to condensation forming on the underside of decks due to cold rain.
The primary purpose of PMR is the protection of the membrane and not necessarily the thermal performance. However it is not so straight forward. While winter heating load increases due to some losses in the R-value for PMR, the summer cooling load is less. The overall impact is going to vary with a number of different factors. A good reference is the following. https://web.ornl.gov/sci/buildings/conf-archive/2010%20B11%20papers/37_Ray.pdf
thank you for the reference. very interesting.